[8.x](backport #4594) Use official serverless product names (#4612)

* use shared attribute (#4594)

(cherry picked from commit f549fed)

# Conflicts:
#	docs/en/serverless/apm-agents/apm-agents-opentelemetry-limitations.asciidoc
#	docs/en/serverless/apm-agents/apm-agents-opentelemetry.asciidoc
#	docs/en/serverless/apm/apm-ui-trace-sample-timeline.asciidoc
#	docs/en/serverless/cases/cases.asciidoc
#	docs/en/serverless/cases/manage-cases-settings.asciidoc
#	docs/en/serverless/index.asciidoc
#	docs/en/serverless/logging/log-monitoring.asciidoc
#	docs/en/serverless/observability-get-started.asciidoc
#	docs/en/serverless/projects/billing.asciidoc
#	docs/en/serverless/reference/elastic-entity-model.asciidoc
#	docs/en/serverless/synthetics/synthetics-get-started-project.asciidoc
#	docs/en/serverless/synthetics/synthetics-settings.asciidoc
#	docs/en/serverless/transclusion/apm/guide/diagrams/apm-otel-architecture.asciidoc
#	docs/en/serverless/transclusion/synthetics/global-managed-paid-for.asciidoc
#	docs/en/serverless/what-is-observability-serverless.asciidoc

* Delete docs/en/serverless directory

---------

Co-authored-by: Colleen McGinnis <[email protected]>
Co-authored-by: github-actions[bot] <github-actions[bot]@users.noreply.github.com>